This invention relates to a method or process of selectively grinding coal to liberate mineral and ash matter entrained with coal and to provide a particle distribution which is in a well defined and relatively narrow range. Modern coal mining and coal cleaning techniques are generating increasing quantities of degraded coal materials. Coal preparation plants produce large quantities of crushed coal and refuse with high water or moisture content. There has been a substantial effort by the Department of Energy to determine effective methods of cleaning coal, of agglomerating the coal into pellets using effective binders. One such agglomeration process is disclosed in U.S. Pat. No. 4,615,712 issued to Wen, Oct. 7, 1986 and assigned to the United States of America as represented by the U.S. Department of Energy, the disclosure of which is incorporated herein by reference.
Traditionally, coal crushed to a size below 150 or even 50 mm has been physically cleaned and is substantially reduced of minerals such as pyrite and ash. Generally, advanced cleaning may be accomplished by physically grinding coal to separate the mineral and ash from the coal particles. Theoretically, fine grinding of coal will result in substantially all of the ash and minerals such as pyrites being free of coal particles.
A problem inherent in grinding of coal is that a portion of the coal in any grinding apparatus is ground to a finer degree than other portions of the coal. For instance, coal passing through a 200 mesh screen may typically contain various particle sizes up to about 80 microns. The particle size distribution may vary considerably but it will be the larger particles for the most part, which contain both pyrite and coal as well as coal and ash. Inefficiencies are introduced in the system by virtue of regrinding coal which is already ash free or pyrite free or coal which has particles in the desired range. Regrinding is expensive and results in particles which are finer than desired and are more difficult to work with later in the process.
Physical coal cleaning involve two distinct steps. The initial size reduction step prepares the coal by crushing, grinding or micronizing in order to liberate the ash and the pyrite. Size reduction by physical crushing produces a mixture consisting of discrete particles of low ash coal and particles high in ash or pyrite. Thereafter, the mixture of discrete particles of low ash coal and the particles with either high ash or pyrite are segregated into clean coal and into refuse product. The separation step may be accomplished by specific gravity separation, froth flotation methods or by agglomeration methods which are known and do not form part of this invention. This invention relates to an improved and more efficient grinding process for producing coal particles having a size distribution in the range of from about 5 microns to about 20 microns and which produces particles which are substantially free of minerals such as pyrites and ash, without excessive grinding.
Heretofore, reduction of coal to micron sizes has been accomplished with an attrition mill which consists of a cylindrical vessel fitted with an agitator and filled with a hard grinding media such as ceramic or iron shot. The feed consisting of a coal slurry and water containing both large and small particles some of which are high in ash and some of which are high in pyrite content is introduced at one end of the vessel. As the agitator turns, the grinding media disintegrates the particles of coal in the feed slurry by attrition. The ground slurry exits at the opposite side of the vessel which is fitted with a screen to prevent the grinding media leaving the mill with the slurry.
The single pass agitator mill method of crushing or grinding coal is unsatisfactory because the ground product may have an average particle size which is acceptable, say in the 20 microns range, but the particles themselves may have a wide size distribution from submicron size to as high as 160 microns. Examination of the particles shows that the finest particle sizes are predominantly made up of ash-free coal and pyrite-free coal, while the coarser sizes contain coal with high ash and pyrite contents. The characteristics of the ground product from attrition mills reduces the efficiency of the micronizing and cleaning operations as well as the dewatering and water treatment steps. Overgrinding increases the difficulty of recovering the products, dewatering the products and clarifying the water for environmental purposes. And, if the high ash and high pyrite-containing coal particles are to be reduced to the required size in a single pass, the entire feed stock must be ground even more finely thereby not only increasing the grinding cost but also aggravating the problems aforesaid.